DESCRIPTION (from applicant's abstract) This proposal addresses fundamental mechanisms in the molecular control of dendritic spine stability. Dendritic spines are micro-specializations of the postsynaptic membrane, and are the prevalent sites of synaptic contact in many mammalian forebrain neurons involved in memory and cognition. Preliminary studies show that dendritic spines are rapidly destabilized following brief exposures to the glutamate agonist NMDA. In vivo synapses are destabilized and lost as part of the normal process of activity-dependent synapse selection. In addition spine loss may constitute an initial stage in excitotoxic neuronal cell death or synaptic decreases related to aging. An understanding of factors that contribute to the relative stabilization and destabilization of spines may therefore facilitate development of therapeutic approaches to intervene a the very earliest stages of the cognitive decline accompanying normal aging or degenerative disease. The proposed studies will utilize fluorescence-based imaging methods to investigate specific hypotheses regarding the mechanism of glutamate-induced spine destabilization. Cultured neurons from rat brain hippocampus will be used as a model system. The temporal and pharmacological properties of glutamate's action on spines will be determined, and the contribution of extra-synaptic factors to spine stability will be explored. Spines are hypothesized to have contractile properties, therefore the roles of actin, actin-binding proteins, and the Rho family of small GTPases will be investigated using immunocytochemistry and microinjection of actin- directed probes. The present investigation will utilize a cell biological approach to determine the molecular mechanism underlying spine regulation.